Exploitation competition in mobile grazers: trade-offs in use of a limited resource

Ecology, March, 1996 by Russell J. Schmitt

INTRODUCTION

Exploitation competition, in which consumers interact indirectly through their influence on shared food items, occurs when both the abundance of the resource is depressed by consumers, and the consumers are limited by the resource. Many experimental studies simply measure the outcome of the combined operation of these two components - that is, the density dependence in some factor related to population growth (Haven 1973, Underwood 1978, 1984, Branch and Branch 1980, Peterson 1980, Peterson and Andre 1980, Brown 1982, Creese and Underwood 1982, Kerfoot et al. 1985, Ortega 1985, Schmitt 1985, Smith et al. 1988). However, knowledge of how consumers affect and are affected by resources can provide a basis for the development of mechanistic models (e.g., Hart 1987, Tilman 1987, 1988, Osenberg 1989, Goldberg 1990, Tilman and Wedin 1991, Sommer 1994). For example, the separate components of exploitation suggest two general means by which two competing organisms can achieve relatively greater individual performance (e.g., reproductive success) when vying for resources: through a greater proficiency to rapidly deplete a shared resource, or by a superior ability to sustain positive growth on a depleted resource (Sommer 1989, Goldberg 1990). Different attributes of an organism may influence its abilities to affect (e.g., Steneck and Watling 1982, Black et al. 1988, Scrimgeour et al. 1991) and respond to resources (e.g., Hart 1981, Dudley and D'Antonio 1991), and it may not be possible for an individual to excel at both (Tilman 1982, 1988, Sommer 1989, Goldberg 1990, Kirk 1991).

While a dichotomy between a high feeding rate and high efficiency in resource use was advanced previously as one aspect underlying r- and K-selection (e.g., Pianka 1970), the more recent mechanistic perspective on trade-offs for obtaining nourishment provides much more explicit, testable hypotheses (e.g., Tilman 1988, Goldberg 1990). For example, D. Tilman (1977, 1982, 1986) developed a body of "steady-state" or "resource-ratio" competition theory that predicts under equilibrium conditions which species will exclude others, as well as conditions that promote coexistence. The theory has been used successfully to predict the outcome of competition in laboratory chemostat experiments on phytoplankton (e.g., Tilman 1977, Holm and Armstrong 1981, Sommer 1983, 1986) and in a field experiment on grasses (Tilman and Wedin 1991). Such a framework can provide especially powerful predictive ability when a universal or unavoidable trade-off is found among different systems, and it can be linked with general features of the consumers (e.g., Tilman 1988, Goldberg 1990).

Despite the fact that exploitation competition appears to be quite prevalent among species of benthic consumers of microalgae (e.g., Haven 1973, Underwood 1976, Black 1977, Brown 1982, Creese and Underwood 1982, Fletcher 1984, McAuliffe 1984, Fletcher and Creese 1985, Ortega 1985, Schmitt 1985, Hart 1987, Osenberg 1989, Kohler 1992; for reviews see Branch 1984, Underwood 1992), it is unclear whether trade-offs to harvest food exist (Chapman and Underwood 1992) or if so, whether they have predictable effects on the interaction. A grazer's feeding structure may play a central role in determining the outcome of exploitation among benthic marine herbivores (Steneck and Watling 1982, Underwood 1984, Hill and Knight 1987, 1988, Black et al. 1988; but see Padilla 1985), and differences in foraging morphologies are known to impose differing effects on the algal resource (Branch and Branch 1980, Lubchenco and Gaines 1981, Hart 1985, Steinman et al. 1987, Steneck et al. 1991, Branch et al. 1992, Kohler 1992). For example, the asymmetry in relative strengths of intra- and interspecific effects for two competing species of limpets was related to differences in their feeding structures that determined how closely microalgae were cropped to the substratum (Underwood and Jernakoff 1981). Similar asymmetries have been reported for several pairs of competing microherbivores (Underwood 1978, 1984, 1992, Branch and Branch 1980, Creese and Underwood 1982, Branch 1984, Fletcher and Creese 1985, Ortega 1985, Schmitt 1985, Kohler 1992; also see Connell 1983, Schoener 1983), yet it is not known whether a single, unifying explanation underlies this apparently common pattern of density dependence among mobile species of benthic grazers.

Exploitation competition among henthic grazers of microalgae rarely leads to exclusion of "inferior" species despite strong competitive effects on the demography (see reviews by Branch 1984, Underwood 1992). Perhaps the closest case is an 80% reduction in local abundance of an intertidal pulmonate limpet caused by resource competition with an acmaeid limpet (Black 1979). If coexistence is the rule rather than the exception in these benthic grazer systems (see Introduction: Known ecological patterns and relationships involving the Tegula species for evidence of coexistence in competing herbivorous snails in the genus Tegula), it is inconsistent with expectations of D. Tilman's resource-ratio theory for species that compete for the same limiting resource.